High-pressure x-ray diffraction data on NaH to 54 GPa at room temperature show a B1(NaCl)-to B2(CsCl) phase transition at PT=29.3 0.9 GPa and volume fraction V/V0=0.61 0.01. In the B1 phase the isothermal bulk modulus B0 is 19.4 2.0 GPa, with a pressure derivative B0 of 4.4 0.5. These results, together with earlier measurements on KH, RbH, and CsH, indicate that the alkali-metal hydrides show the same strong dependence of PT on cation as do the alkali-metal halides. The systematically lower bulk moduli of the alkali-metal hydrides as compared to the alkali-metal halides can be attributed to incomplete charge transfer in this series of partly ionic compounds. © 1987 The American Physical Society.